A hybrid vehicle may provide regenerative driveline braking to slow a vehicle. During regenerative braking, a vehicle's kinetic energy may be converted to electrical energy. The electrical energy is stored in an energy storage device where it may be held until it is needed to supply power to the vehicle or perform some other function. The energy storage device may have capacity constraints so that it may store a limited amount of electrical energy. Consequently, it may not be desirable to continue to convert the vehicle's kinetic energy into electrical energy even though conditions are favorable to produce electrical energy. The driveline may transition from a regeneration mode to an engine braking mode to maintain driveline braking when it is not desirable to continue regeneration. However, transitioning from regenerative braking to engine braking may produce driveline torque disturbances. Driveline torque disturbances may be especially noticeable if engine rotation has been stopped before engine braking is initiated.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for controlling driveline braking, comprising: providing driveline braking via an electric machine while rotation of an engine is stopped; operating the electric machine in a speed control mode in response to a request to provide driveline braking via the engine; starting the engine; accelerating the engine to a speed of the electric machine; and closing an open driveline disconnect clutch in response to engine speed substantially equaling electric machine speed.
By operating an electric machine in a speed control mode during a transition from regenerative braking to engine braking, it may be possible to reduce driveline torque disturbances. In particular, an electric machine may be operated in a speed control mode so that the electric machine can react to driveline speed variations caused by the torque variation. In other words, a torque change in the driveline may be reflected in the speed of the electric machine, and torque output of the electric machine may be adjusted to counter act torque that caused the change in driveline speed. In this way, driveline torque variation may be reduced so that driveline speed follows at a desired driveline speed.
The present description may provide several advantages. Specifically, the approach may reduce driveline torque disturbances of a hybrid driveline. Additionally, the approach may improve vehicle drivability.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.